Known in the art is an internal combustion engine arranging an NOx selective reduction catalyst in an engine exhaust passage, arranging an NOx storage catalyst storing NOx contained in the exhaust gas in the form of nitrates and releasing the stored NOx when feeding a reducing agent in the engine exhaust passage upstream of the NOx selective reduction catalyst, and feeding urea to the NOx selective reduction catalyst and using the ammonia generated from the urea to selectively reduce the NOx contained in the exhaust gas (for example, see Japanese Patent Publication (A) No. 2005-2925). In this internal combustion engine, the amount of urea feed is determined considering the NOx amount stored in the NOx storage catalyst and the NOx amount released from the NOx storage catalyst. For example, when the NOx storage catalyst releases the NOx, the amount of urea feed is increased by exactly the amount of reduction of the NOx amount released.
However, in this internal combustion engine, when feeding the reducing agent, that is, the fuel, for releasing the NOx from the NOx storage catalyst, part of the stored NOx is released in the form of NO or NO2 from the NOx storage catalyst, but part of the stored NOx is further reduced more than NO and is released in the form of ammonia NH3. In this case, it is not clear to what extent the stored NOx is released in the form of NOx and to what extent it is released in the form of ammonia NH3. In this case, if the amount released in the form of ammonia NH3 is large, the released NOx is reduced by this ammonia NH3, so it is no longer necessary to increase the amount of urea feed.
In this regard, the above-mentioned internal combustion engine is predicated on all of the stored NOx being released in the form of NOx. Therefore, there is the problem that since the amount of urea feed is increased by exactly the amount of reduction of the NOx amount released, the amount of urea feed ends up becoming excessive. Such a problem occurs as long as a reducing agent is used to make the NOx storage catalyst release NOx.